This application is based on and claims priorities from Japanese Patent Application Nos. Hei. 3-339270 filed Oct. 24, 1991, Hei. 3-349295 filed Oct. 31, 1992, Hei. 3-349296 filed Oct. 31, 1991 and Hei. 3-349297 filed Oct. 31, 1991, the disclosures of which are incorporated by reference herein.
The present invention relates to a roof surface measuring apparatus for measuring the accuracy of a roof surface of a rectangular prism and a roof surface formed by joining surfaces of two mirrors, etc. substantially perpendicularly (roof surfaces for optical parts etc.).
In the finder system of some types of recent lightweight single-lens reflex cameras, a penta mirror, which is a combination of mirrors, is used instead of a penta prism. A roof mirror constitutes Dart of such a penta mirror.
Requirements for a highly accurate penta mirror include that the ridge line of the roof mirror not have a width, and that an angle formed by the two mirror surfaces be precisely 90.degree.. Conventionally, in the case of roof prisms, such as a penta prism, that are produced by polishing a glass material, the flatness of the roof surface is assured in a wide area. Therefore, the angle of the roof surface can be approximated by measuring angles in this wide area, and the angle of the roof surface is measured with an autocollimator.
Where the roof surface is manufactured by, for instance, plastic injection molding, unevenness such as a bend and rounding may be caused in the mirror surface by local strains occurring during the molding.
By conventional measuring methods, it is difficult to measure the roof angle of roof surfaces having insufficient flatness, particularly roof surfaces having insufficient flatness in the vicinity of their ridge line.
In the case of performing measurements on the roof surface using the interferometer, the contrast of an interference fringe varies with the reflectivity that depends on the material of the roof surface itself and the medium before the roof surface. Therefore different contrasts of the interference fringe are obtained for a roof surface of a roof mirror and that of a penta prism even with the same light incident thereon.
However, since the contrast variation with the material of the roof surface is not considered in the above conventional interferometer, with certain kinds of materials of the roof surface the contrast of the interference fringe is unduly reduced to make the observation very difficult.
In the case of performing measurements on the roof surface using the interferometer or the like, it is necessary to detect the position of the ridge line precisely. This is particularly important for molded roof mirrors because the bend of the reflection surface occurs irregularly.
However, the correct detection of the ridge line position by the above conventional measuring method becomes more difficult as the roof surface is manufactured more precisely, i.e., as the ridge line becomes more like a straight line and its width approaches zero.